Radio tuning system



Sept. 15, 1942!] M FARFL ETAL 7 2,296,093

A RADIO TUNING SYSTEM File d June 1. 1940 I wi Q Gttomeg signal input and interstage Patented Sept. '15, 1942 UNITED STATES PATIENT OFFICE A RADIO TUNING SYSTEM Victor M. Farrel, Camden, and Jerome C. Smith,

Haddon Heights, N. L, assignors to Radio Corporation of America, a corporation of Delaware Application June 1, 1940, Serial No. 338,320

' 2 Claims. (cilia-44) This invention relates to radio tunin'g systems, and has for its primary object to provide an improved variable permeability tuning system for radio receivers and the like.

It is also an object of the invention to provide an improved tuning system of the variable perme- I ability type for a plurality of tuned coupled signal circuits, whereby the coupling and the gain therethrough in a predetermined variable tuning range 1 may effectively be controlled.

It is a further object of the present invention to provide an improved variable permeability tuning system for superhetercdyne receivers which operates to maintain the image response at a relatively low level throughout the tuning range.

The invention will further be understood from the following description, when considered in connection with the accompanying drawing, and its scope is pointed out in the appended claims.

In the drawing, the figure is a schematic circuit diagram of the r-f amplifier and first detector of a superheterodyne receiver, provided with a variable permeability tuning system and coupling means embodying the invention.

Referring to the drawing, an r-f amplifier stage comprising a high impedance amplifier tube 5 such as a pentode, having a signal input grid 5, a cathode 1, an-output anode 8, and suitable screen and suppressor grids 9 and Ill, respective-- ly, is coupled, through a variable permeability tuned input circuit II with a' similar variable permeability tuned antenna or signal supply circuit H.

The first detector or converter stage, following the r=f amplifier stage in the signal channel 01 the receiving system, comprises a combined detector and oscillator tube l3 having a signal input grid l4, a cathode I5, a screen grid It for the signal input grid, anoutput anode l1, and oscillator grid'and anode electrodes l8 and I! respectively, although separate tubes for the two functions may be used.

The signal input grid I4 isconnected with a tunable input circuit which is coupled with a tunable output anode circuit 2| for the r-! amplifler stage 5. The oscillator grid I8 is connected I with a tunable oscillator circuit 22 comprising a tuning inductance '23 having a. movable terroma gnetic tuning core indicated at 24, and tuning capacitors 24 and 26.

The low potential side of the oscillator circuit is connected to. ground or chassis 21, while the high potential" side thereof is coupled to the grid II with a suitable grid leak resistor 29' between the I grid l8 and ground 21.

coupling circuits of The tunable r-I signal input circuit 20 for the detector comprises a similar tuning inductance 30 having a movable term-magnetic core indicated at 3| and a shunt tuning capacitor 32 0011-! nected to ground as indicated at 33. The circuit is completed from the low potential side of the winding to ground through a coupling capacitor 34 and a series resistor 35. The capacitor 34 has substantially negligible impedance in the frequency range of the circuit 20.

The output anode circuit 2 is tuned in a similar manner by a variable inductance 38 having a prevent undesired stray coupling. In the present movable form-magnetic tuning core indicated at 39 and suitable capacity means as hereinafter referred to. Themutual inductance or coupling between the inductance 38 and the inductance 30 is preferably low or substantially zero. The resistor 35 is included in both the output circuit 2| of the r-i amplifier 5 and the input circuit 20 of the first detector as a common coupling impedance, the circuit 20 including the resistor serially. therein as above pointed out and the inductance 38 being connected in circuit therewith through a bypass capacitor 40 and acoupling.

capacitor 4|. The capacitors are therefore included in the circuit 2| across the inductance 3'8. Anode potential is supplied to the anode circuit 2| through a filter resistor or impedance 43 and bias potentials-are supplied to the signal grid I4 through a connection including a resistor or impedance 44.

While no shielding is indicated in the schematic circuit, it should be understood that the various elements may be suitably shielded or spaced to example, inductive coupling between the circuits II and I2 andbetween the windings 30 and 38 is minimized by spacing, or shielding, as desired.

The impedance 0! variable inductances 30 and It increases with reduction in the frequency response of the circuits tuned thereby as the tuningcores 3| and 39 are moved into the windings, so that the common coupling impedance 35 becomes a smaller percentage of the total circuit impedance in each of the circuits 20 and 2| as the said circuits are tuned to lower frequencies in the tuning range. In the present example, this range may be assumed to be the broadcast signal receiving band-oi substantially 550 to 1500 kcs. for both circuits 20 and 2| as well as circuits H and I2.

through the usual grid capacitor 28 n connection Therefore, it will be seen that as the impedance of the inductances in the tuned circuits increases as the circuits are tuned toward the lower end of the tuning range, the resulting tendency for the gain to increase is counteracted by the effective decrease in coupling through the impedance 35.

By suitably relating the resistance or impedance of the common coupling element 35 with the impedance of the tuning inductances 30 and 38, as by adjusting the value of the resistor 35, it has been found that the gain throughout the tuning range may be made substantially constant between the r-f amplifier stage 5 and the input circuit of the detector Hi. The low pass filter arrangement of the circuit is advantageous in suppressing image frequency interference.

Referring now to the r-f input circuit II and the signal supply circuit l2 on the input side of the r-f amplifier stage 5, it will be seen that these circuits are similar to circuits 20 and 2|,

I ductance 52 having a movable tuning core of ferro-magnetic material, as indicated at 53, together with a shunt tuning capacitor 54 connected to ground 55, the capacitor 46 and a blocking capacitor 56 to the low potential side of the winding 52. The coupling or mutual inductance between the inductances 41 and 52 preferably is substantially zero as in the case of the output coupling network.

As in the corresponding input circuit 20 for the detector, grid bias is supplied to the grid 6, which is connected with the circuit l I, through an impedance or resistor 58.

The tuning cores for' the various circuits are interconnected, as indicated by the dotted line 59--60, for unitary movement and control, whereby all of the circuits may be tuned simultaneously by common means to provide tuning response to a desired signal, the oscillator being tuned preferably above the r-f and-detector circuits by the amount of the intermediate frequency.

The intermediate frequency signals are derived from the output anode'clrcuit SI of the detector and applied to a suitable intermediate frequency amplifier circuit, of which the input coupling means is shown as comprising a tuned interstage coupling transformer 62. The remainder of the superheterodyne receiver is not shown as it does not concern the invention.

While the common circuit impedance 35 in the r-f circuits 20 and 2| tends to maintain substantially constant gain in the coupling between the r-f amplifier stage and the first detector, the common coupling capacitive reactance 46, in the input or antenna circuits II and I2, tends to maintain substantially constant coupling between the said circuits.

Since the capacity element 46 is fixed and the inductance elements 41 and 52 are the variable elements, theelement 46 has the common series capacitive reactance may be fixed at such a value that the coupling is optimum between the circuits II and I2 and remains substantially constant at any predetermined value throughout the tuning range.

Furthermore, by providing the type of coupling shown, two tuned circuits may be provided between the signal supply source and the r-f amplifier stage or other input stage of the receiver to improve the image response appreciably over that heretofore provided in receiving systems. For example, in the system shown, it has been found that the signal-to-image response at the high frequency end of the tuning range may be of the order of 20000 to 1, and of the order of 175,000 to 1 at the low frequency end of the tuning range, as compared with the usual variable capacity-tuned circuits in which the signalto-image response at the high frequency end of the tuning range may be of the order of 1,000 to 1 and of the order of 12,000 to 1 at the low frequency end of the tuning range.

In the case of a double tuned circuit in the;

antenna or input stage and a double tuned circuit in the-r-f stage, the first circuit gives a more :uniform gain .in the antenna stage while the second circuit provides a more uniform gain in the r-f stage particularly in connection with a high impedance amplifier tube such as a pentode.

It will be appreciated that the inductance and capacity relation in the circuits may be reversed to obtain a high or low pass filter effect. For example, in the circuit shown a low pass filter effect is provided in the. r-f stage in the coupling network 202I which is effective to attenuate the undesired image frequency response.

We claim as our invention:

1. In a variable permeability tuning system, the combination with a high impedance amplifier tube, of a tunable signal input network therefor comprising two tunable signal conveying circuits, a variable permeability tuning inductance in each of said circuits, a common capacitive reactance device in each of said circuits at the low-potential side thereof providing a predetermined constant coupling between said circuits throughout the tuning range thereof, and a. tunable signal output network for said amplifier tube comprising a variable permeability tuning inductance, a fixed tuning capacitor and a series resistor, said resistor having a resistance value such that the gain through said circuit across said resistor as a coupling element for signal output is substantially constant throughouta predetermined tuning range.

2. In a variable permeability tuning system, the combination with a high impedance amplifier tube, of a tunable signal input network therefor comprising two tunable signal conveying circuits, a variable permeability tuning inductance in each of said circuits, a common capacitive reactance device in each of said circuits at the low potential side thereof providing a predetermined constant coupling between said vrc'roa M. FARFEL. JEROME 0. SMITH. 

